Titanium dioxide pigments and fillers

ABSTRACT

A method of modifying the surface properties of titanium dioxide pigments and fillers by forming on the particles a surface coating of an aluminium compound, the coating having a significant number of acidic sites with pKa values of less than 2.8.

United States Patent Hodgkin et a1.

[ Dec. 30, 1975 TITANIUM DIOXIDE PIGMENTS AND FILLERS Henry Solomon, Glen Waverley, all

of Australia Assignee:

Commonwealth Scientific &

Industrial Research Organization, Campbell, Australia Filed:

Feb. 19, 1974 Appl. No.: 443,795

Related US. Application Data Division of Ser. No. 224,606, Feb. 8, 1972, Pat. No.

Foreign Application Priority Data Feb. 10, 1971 Feb. 10, 1971 Feb. 10, 1971 Feb. 10, 1971 Australia 3979/71 Australia 3981/71 Australia 3983/71 Australia 3985/71 [52] US. Cl. 106/300; 106/308 Q; 106/308 N; 117/100 B; 260/42.l4; 260/42.15; 260/42.53 .[51] Int. Cl. C09c l/36 [58] Field of Search 106/308 Q, 300, 308 N; 260/42.14, 42.15, 42.53; 117/100 B [56] References Cited UNITED STATES PATENTS 3,004,858 10/1961 Sheehan et al. 106/300 3,224,993 12/1965 Wynne 106/300 3,451,835 6/1969 Ganter et al 106/308 Q 3,728,142 4/1973 Rudolph et al. 106/300 Primary ExaminerWinston A. Douglas Assistant Examiner-J. V. Howard Attorney, Agent, or Firm-Schuyler, Birch, Swindler, McKie & Beckett ABSTRACT A method of modifying the surface properties of titanium dioxide pigments and fillers by forming on the particles a surface coating of an aluminium compound, the coating having a significant number of acidic sites with pKa values of less than 2.8.

11 Claims, N0 Drawings TITANIUM DIOXIDE PIGMENTS AND FILLERS This is a division of application Ser. No. 224,606,

filed Feb. 8, 1972, now US. Pat. No. 3,834,923.

This invention relates to a method of modifying the surface properties of particulate mineral materials, specifically titanium dioxide and to particulate titanium dioxide having modified surfaces. In particular the invention is concerned with such materials having acidic surfaces (as hereinafter defined) and to methods for their manufacture and modification to produce pigments or fillers having unique and improved properties. More specifically the invention is concerned with methods whereby the organophilic/hydrophilic and reinforcing properties of titanium dioxide pigments or fillers can be modified to suit the medium in which the pigment or filler is to be used.

The prior art refers to the coating of particles of titanium dioxide pigments with substances variously referred to as aluminum oxide, silicate, silicaalumina and other similar terms, and many such coated pigments are available commercially. Such prior art places emphasis on the desirability of (a) essentially neutral surfaces and (b) a minimum level of photochemical activity of the coated pigment. These pigments are also known to be non-reinforcing that is, their incorporation in plastics media usually results in compositions of increased hardness and stiffness, but without increased tensile strength or toughness.

It is also well known in the art that modification of a mineral pigment or filler with organic compounds, including amines (a specific example of a base), changes its surface properties but the surface of the gel-coated or natural pigment is hydrophilic and neutral and hence interaction with the organic modifying agent is weak. Only relatively hydrophilic modifying agents can be used. In addition, the original gel-coating influences, usually in an adverse manner, the properties of the final modified material, particularly in exhibiting a tendency to flocculate in non-aqueous systems.

The present invention however has as its main aim the provision of acidic surface coating on titanium dioxide which may optionally be further modified to provide materials which have unexpectedly useful reinforcing and organophilic properties which will be described hereinafter.

It is also known from the prior art that the surface of certain natural alumina-silicate materials, notably the kaolin group clays, can be modified to give an acidic reaction by means of a procedure involving heat treatment. However, this behavior is not known for titanium dioxide.

Before defining the present invention it is necessary to define the term acidity as used herein. Acidity is used in the broad sense to describe the ability to the surface to donate protons to a suitable base (i.e. Bronsted acidity) or to accept an electron, or electrons usually as a electron pair, from a suitable organic base (i.e. Lewis acidity). The acidity can be considered in terms of the number of acid sites produced on the surface of thematerial and their acid strength. We measure both the number and the strength of the surface acid sites by titration of the surface with butylamine in a non-aqueous solvent using a series of organic indicators, usually referred to as Hammett indicators. These indicators have differing pKa values and consequently enable a distribution of sites with various acidities to be obtained. The method used (which is described in the literature) is as follows:

To a sample of the material (4.000 g) dry benzene (20.0 ml) is added. This slurry is then slowly titrated with a benzene solution of butylamine (0.02 millimolar) using Hammett indicators as external indicators. Thorough stirring is maintained during the titration. Samples of commercial products are tested as received. Samples prepared in the laboratory are allowed to stand for at least two weeks before testing, to condition and thus be more equivalent to commercial products.

Typical Hammett indicators which can be used for determining the number and strength of acid sites are listed below with their pKa values.

increasing order of acid strength A considerable number of commercially available titanias have been tested by us to determine the extent of acid sites on the surface of these materials.

-contmued Manufacturer Grade Manufacturer Grade TCL R-HD6 R-CR7 National Lead Titanox CL Titanox CLNC American Potash Tronox CR-800 Titanox RA-67 1 (IR-801 Titanox RA-Sl n Titanox RA-47 Titanox RA-44 Bayer R-FDl R-FD2 R-KBI New Jersey Zinc R-760 R-KB2 R4770 R-KB3 R-UF Thann et Mulhouse RL-l7 Titangesellschaft Xronos R Xronos RN-4S In all cases, the acidity found using the titration method described above, titrating with butylamine. in benzene to a pKa value of 2.8 was nil. In other words, these materials have no sites with an acidity of less than 2.8.

We have now found that titanium dioxide particles can be provided with surface coatings having significant numbers of acid sites with pKa values measured as described above of less than 2.8. Such materials have unexpectedly useful anddesirable properties.

In accordance with, one aspect of this, invention, therefore, we provide a pigment or filler material consisting of particulate titanium dioxide having a surface coating comprising an aluminium compound and having a significant number of acidic sites with a pKa value of less than 2.8. j

Significant" in this context means that the presence of the sites can be detected and their strength measured by the methods described herein.

The titanium oxide may be in the form of either rutile or anatase and optionally may have a pre-coating applied to it. t

The aluminum compound is preferably an alumino silicate material, most preferably formed by'the methods hereinafter described. As indicated above coatings of alumino-silicate materials are known per se but not with acidities of the defined level.

In accordance with another aspect the invention provides a method for producing an acidic surface coating on a particulate titanium dioxide pigment or filler material which comprises the steps of:

a. forming a coating of an aluminium compound on the surface of the particles;

b. forming the coated particles into a slurry in an aqueous medium at pH of less than 5.0;

c. recovering the particles from the slurry; and

d. heating the particles; thereby to produce particles having a significant number of acidic sites in the surface thereof with a pKa value of less than 2.8.

The titanium dioxide may be in the form of either rutile or anatase.

In accordance with a further aspect of the present invention a particulate titanium dioxide pigment or filler having a surface coating with a significant number of acidic sites with a pKa of 2.8 or less (measured as described hereinafter) is treated with an organic base (as hereinafter defined). The acidic titanium dioxide materials of the present invention are preferred but titanium dioxide of like acidity from any other source may also be used.

The organic bases which can be used are not restricted to any specific class but rathercan be chosen from the general classesof organic compounds with basic properties. Accordingly, the term -organic base is used herein in the broad sense to include materials which bond by salt formation, i.e. by acception a proton from the surface, and materials which donate electrons, or electron pairs, to the surface. The latter are often referred to as Lewis bases.

Thusthe organic bases may be selected from the classes comprising amines, alcohols, esters, ketones, ethers and other classes of compounds which contain oxygen or nitrogen which will act as proton acceptors or electron donors. The classes of compound may be selected from the aromatic or aliphatic series, with straight or branched chain and the compounds may be substituted with other groups such as, halogen or nitro, provided the compound retains its basic properties, Amines and alcohols are particularly suited to-producing desirable modifications to mineral surfaces by this process. t j I A wide range of, amines and alcoholsis suitable for use in this process, including primary, secondary and tertiary amines or alcohols. In specific cases, polymeric compounds with suitable functional groups may be used to modify the acidic surface, for example Versamids or polyamines with terminal amino groups give useful products. Amines of naturally occurring materials, such as tallow amines, are also useful. Unsaturated amines or alcohols are particularly useful as they can facilitate bonding of the pigment or filler to polymeric matrices by free radial or other reactions during subsequent compounding, curing or drying.

The products obtained from the process just described also form a part of this invention. They are ideally suited as pigments and fillers for use in organic polymer compositions, including polyesters, polyurethanes, rubbers, polyvinyl chloride and polyethylene type compositions. Such compositions containing the above products are also a part of this invention.

The modified materials of this invention, when used in such polymer compositions, exhibit the following improved properties:

The materials disperse more quickly and better in non-aqueous systems.

When dispersed in aqueous and non-aqueous systems or solutions of organic polymers, the viscosity is significantly lower than with commercial materials. This effect is often readily changed by suitable selection of the base. Thus, one base'may lower the viscosity of the material in a polyester system but not in rubber, and another may produce the opposite effect.

A high bulking factor is often shown by the treated materials, which is an indication of the extremely good dispersion obtained by this process.

The treatment makes the surface moreorganophilic and hence more hydrophobic. Thus the'wetting properties of the material in aqueous and non-aqueous systems can be modified to a desired level.

The materials resist the absorption of moisture from the air.

According to a still further aspect of the present invention a particulate titanium dioxide pigment or filler having an acidic coating. (as hereinbefore described) is provided. with an organic polymeric coating produced by treating the pigment or filler with a polymerisable monomer or mixture of monomers under conditions such that polymerisation of the monomer or mixture of monomers is catalysed by the pigment or filler surface and a tightly bound polymer coating is formed on the surface, which coating contains or can be treated to yield reactive groups capable. of initiating grafting reactions between the coating around the pigmentor filler surface and a matrix polymer in which the pigment or filler is dispersed.

The monomers which can be used are those which are susceptible to cationic polymerization, which the acidic surfaces of the pigment or filler will promote.

The reactive groups are preferably but not essentially also capable of crosslinking coating polymer and the matrix polymer in the vicinity of the pigment or filler particle surface. For example, the coating polymer may contain groups which can be oxidised or otherwise converted to yield peroxidic or other radical-generating species which are capable of initiating the necessary grafting reactions. Typically such monomers include sytrene and substituted styrenes,. including styrenes substituted with p-methoxy or p-methyl groups; aliphatic and aromatic olefins withmore than one double bond; vinyl ethers, including methyl, butyl and 2-ethylhexyl vinyl ethers; chloro substituted vinyl ethers; N- vinyl carbazole, divinylbenzene, vinyl ketones; vinyl ethers with general substituted groups, including linoleyl, allyl; indenes, coumarones; vinyl phenyl siloxanes; a-methyl styrene and it derivatives.

Polymers formed from such monomers can be reacted, if necessary in the presence of catalysts or actinic light, with'oxygen, ozone or other oxidizing species capable of converting the reactive groups contained in the polymers, to peroxides, hydroperoxides, ozonides, or other species which can actas radical generators.

The amount of coating material applied to the particulate-mineral may varyin amount from about 0.5% to 10.0%, preferably 1% to by weight of filler or pig ment.

The steps of the process defined abovewill now be considered separately in detail.

a. Coating. of titanium dioxide .particles.-

The formation of aluminum compounds as surface coatings of this general type is well known in the art. The exact nature of the coatings is not known, particularly after the coating material has been dried. Such coatings are referred to loosely in the literature as aluminium oxide, silicate, Al O /SiO and similar terms. These are veryoften in hydrated form. Many commercially available materials are already coated in such a way and can be used for the following steps of the method of this invention without further treatment.

The order and manner in which the aluminum (and silicon) compounds are deposited are not critical, although certain procedures work better with some pigments: Similarly it is sometimes necessary or desirable to subject the coated pigment or filler to a treatment, such as a heat treatment, prior to pH adjustment and heating. The coating step-may thusbe carried out by slurrying the titania in water and adding a solution of the'fcoating materials, for example, by adding the solution of a soluble aluminum salt such as aluminum sulphate. If an aluminum silicate type of coating is being prepared, then this may be done in any convenient way.

A solution of an aluminum salt, such as aluminum sulphate or sodium aluminate, may be added first and then a silicate solution added thereafter. Alternatively the order can be reversedor the two solutions added together or in small increments alternately. If desired, the salt or salts may be added in solid form. Since the coating is not. meant to correspond exactly to aluminum silicate, the molar ratio of silicon compound to aluminum compound can vary from 0 1 up to 5 1. Although in general the higher the amount of aluminum compound the greater the total audity.

Some attention to the concentration of the added salts is required to ensure that a satisfactory gel coat is obtained such precautions are known art.

The titania used as starting material will normally be a finely divided, particulate material. It can be any of the commercially available materials, either alreadly coated or without coating. The acidity produced in these materials may vary according to the nature of the starting material its pretreatment (if any), and particularly its crystal shape and particle size. The smaller the particle size the greater the number of acid sites that will be produced by the process, because of the larger surface area exposed to the process.

Normallythe coated titania is filtered off and washed with water before proceeding to the next step of the process. However,-in some cases an adjustment to the pH can be made direct to the slurry during and/or at the conclusion of the coating procedure. It is also sometimesadvantageous to calcine the coated material before proceeding to the second step of the process.

b. pH adjustment The concentration of theslurry is not critical but in practice is limited by difficulties in .stirring, if it is too concentrated,,and cost, if it is too dilute. If desired, the acid solution may be prepared first to a pH of-less than 5 and the solid added to it, with further adjustments to the pH if necessary.

.The pH to which the slurry must be adjusted will vary according to the type of titanium dioxide and the acidity, desired inthe material. The optimum pH is between Sand 3 although. low pH values can be used if the treatment time is short.

The adjustment to pH may be brought about by the addition of anywater soluble acidic materiaL Thus an acid or an acidic salt or oxide .may be employed for this purpose. The acid used can be organic or inorganic. Mixturesof acids, either inorganic or organic or both, are included. -Typically, sulphuric acid, hydrochloric acid, phosphoric acid acetic acid, formic acid and other aliphatic carboxylic acids, benzoic acid and other organic armoatic acids, may be employed.Acid salts such as sodium hydrogen sulphate or aluminium potassium sulphate, or acidic oxides, such as sulphur trioxide may be used.

0. Heat treatment I After recovery of the coatedparticles, for example by dewatering and drying, the final heating can be carried out in any suitable equipment between the temperatures of about-l and 800C. The best results are usually obtained by heating in the range of 120 300C. In applications wheremore highly acidic sites are required higher temperatures can be an advantage. The product should be heated for sufficient time to produce the acidity desired on the surface. The time required will depend on theequipment used, but must be sufficient to remove free water adhering to the material and then to convert the coating to produce the desired acidic sites. It may be extended for longer periods without adversely affecting the'properties of the product.

The final product may require size selection where a small amount of agglomeration results from the heat treatment.

The above-described operations result in materials with significant numbers of sites with acidities less than 2.8 and'hence improved properties. As will be seen from the following examples, significant acidity is produced in all cases when treated according to this invention. Furthermore, by testing each material with progressively more acid Hammett indicators, the acid strength of the sites can be estimated. it is found that some sites with acid strengths in the pKa 3.0 to 5.6 range are present in most cases but the majority are in the range +1.5 to 3.0. The commercial materials tested, as mentioned above, have no sites less than 2.8, and only four commercial materials tested have sites in the 40m 2.8 range; the very large majority have no sites more acid than pKa 4.0.

The acidic titanias are useful in all of the areas of application in which the materials, without the treatment of this invention, are used. Thus they are useful as pigments or fillers in aqueous and non-aqueous media. They are of particular use in paper coatings, paints, plastics, elastomers and various specialized applications such as pressure sensitive copy paper where the acidity can be used to cause colour reactions. They are also of interest when further modified, as described below by reaction with basic materials or by grafting polymers onto their surfaces.

d. Treatment with organic base.

In general, the method of this invention involves the bringing together in any suitable manner of the prepared acidic titanium dioxide and the organic base.

Thus the two components can be physically blended or mixed in any suitable equipment without the addition or use of any other component. The choice of equipment will vary according to the process conditions and product requirements, e.g. whether the base component is liquid or solid, the amount of base to be added, and so on. Typically one can use a rotary blender, centrifugal mixer, ribbon blender, vertical mixer, ball mill or a rod mill. A particularly useful technique involves the use of a fluidized bed mixer in which the base is added to the fluidized mineral. If desired, additional components can be mixed or blended with the product before, during or after the incorporation of the organic base.

8 A very convenient method for achieving uniform reaction of the organic base consists in the use of a suitable non-aqueous solvent in which the mineral component can beslurried. The organic base'component can be added to the slurry either incrementally or continuously, alone or dissolved in the same or different solvent. Alternatively, the organic base may be dissolved in the solvent and the mineral then added to form a slurry. Typical solvents which are suitable are benzene and similar aromatic hydrocarbons, liquid paraffin hydrocarbons such as hexane, heptane and higher homologues, petroleum fractions, kerosene, halogenated solvents such as trichlorethylene, carbon tetrachloride, ethers and esters. In selecting the solvent, a'prime consideration is that the interaction between the solvent and the pigment or filler surface must be less than between the organic base and the pigment or filler surface. In other words, the solvent should be less basic than the modifying agent.

Water, attracted to the acidic sites of the titanium dioxide, may interfere with the interaction with a base. It is therefore often desirable to dry the material before use, say at C, in order to reduce'the amount of water bonded to the surface.

In general, amounts of organic base up to about 5% can be used, but the optimum concentration of base cannot be determined by any general rule. With polymeric bases, greater amounts of the base can generally be used. The surface area and particle size of the mineral will affect the amount of organic base to be used. In general terms, the larger the surface area and'the smaller the particle size, the greater the amount of base required to achieve the desirable treatment.

e. Production of polymeric coating.

Preferably the polymerisation is carried out by contacting the monomer with the acidic pigment or filler in such a way that the particles are each coated with polymer but remain separate. For example, the pigment or filler can be fluidized and the monomer introduced into the fluidized system as a vapour at a suitable temperature. Alternatively, the monomer may be added to the fluidized bed as a solution in a volatile solvent which evaporates from the bed.

It is also possible to disperse the mineral in a solvent which does not interfere with the catalytic properties of the surface of the mineral and in which the monomer is dissolved or to which the monomer is added, continuously or incrementally. Typical solvents are benzene and similar aromatic hydrocarbons, liquid paraffin hydrocarbons such as hexane, heptane and higher homologues, petroleum fractions and kerosene. Adequate stirring, agitation or mixing is required to maintain the mineral as a dispersion of separate particles on which the polymerization will take place. In some cases, small amounts of dispersing agents which do not interfere with the polymerization may be added.

Formation of radical inducing species in the coating polymers may be carried out using the methods described hereinafter. Thus this step can be achieved by passing air, oxygen or ozonised air or oxygen-through the dry, coated filler or pigment at or near room temperature. Thorough mixing can be ensured by the' use of a fluid bed or tumbling mixer and the extent of peroxide formation can be determined by iodometric titration; It is often desirable to expose the coated materials to the vapour of polar compounds such as water, alcohols or esters prior to oxidation in order to increase the storage life of the oxidized product, which may be adversely affected by the strongly acidic surface.

Controlled amounts of crosslinking of the polymer coatings can be introduced where necessary, in order to prevent desorption of the coating. This can be achieved in two ways:

a. by the use of divinyl monomers mixed with another monomer, e.g. divinyl benzene/styrene, or

b. by the decomposition of some of the peroxidic groups produced on the surface. The cross-linking process may occur during the incorporation of the coated pigment or filler into a plastics composition.

Plastics and elastomeric compositions containing the coated fillers or pigment can be prepared by mixing the matrix polymer and coated pigment or filler using conventional hot processing equipment well known in the plastics art. It is preferable, although not essential that the finely divided ingredients be dry blended and then mixed in a Banbury or Brabender Plastograph mixer. The ratio of coated filler to matrix polymer is a matter of choice and the proportion of coated material may comprise from 1 to 100% of the composition. Other ingredients such as plastics, dyes, additional cross-linking agents, etc., may be incorporated in the composition. The compounding conditions are dependent on the nature of the coating material and the matrix polymer, although in the absence of added accelerators a typical peroxidised coating would require compounding at 100 to 200C for 5 minutes duration for completion of the coating-matrix polymer reaction. Alternatively the ingredients may be compounded under such conditions that some of the radical-generating species remain intact and the radical induced reactions completed in a subsequent curing process.

The organic material coating the filler or pigment particles preferably comprises one or more groups which can be oxidized or otherwise converted to yield peroxidic or other radical-generating species which are capable of initiating the necessary grafting reactions. These groups can readily be incorporated into the coating material by appropriatechoice of monomers used to prepare the coating. Such reactive groups include: allylic, aldehydic, benzylic, secondary or tertiary hydrogen atoms, or aromatic, ethylenic or acetylenic unsaturation, or other groups capable of reaction, if necessary in the presence of catalysts or actinic light, with oxygen, ozone or other oxidising species capable of converting these reactive groups to peroxides, hydroperoxides, ozonides, or other species which can act as radical generators. Alternatively, the coating material may contain functional groups which can form adducts with peroxides, hydroperoxides or substituted peroxide derivatives or other species, capable of acting as radical generators.

As a further alternative, the coating material may contain as reactive groups species which can initiate grafting and/or crosslinking reactions between the coating and matrix polymers or other ingredients of the plastics or elastomer composition by other than radical-induced processes or the coating may contain groups capable of conversion to such reactive species.

In addition to the radical generators or their precursors, the coating material may contain reactive groups including vinylic or substituted vinylic groups, capable of undergoing crosslinking reactions either during the coating process, during subsequent treatment of the coated pigment or filler, or during the compounding or 10 curing of materials incorporating the coated pigment or filler. 1

The coating materials may also contain functional groups specifically incorporated to promote adhesion of the coating to the pigment or filler surface by the formation of strong chemical or physical bonds.

The coating materials include, but are not necessarily confined to auto-oxidisable drying oils, including wood, linseed, safflower and soya oils and other unsaturated species including-cyclopentadiene and dicyclopentadiene, butadiene, terpenes, and their derivatives which may form polymeric peroxides on exposure to atmospheric oxygen, as well as alkyd or other resins of natural or synthetic origin which may contain these or similar auto-oxidisable species; vinyl aromatic polymers and copolymers, including those of styrene, methoxy styrene, vinyl toluene, ethyl vinyl benzene and divinyl benzene; alkene, alkadiene polymers and copolymers which contain residual unsaturation or oxidisable centres; polyesters, polyethers, acrylic, substituted acrylic and other vinylic or condensation polymers which contain residual unsaturation or oxidisable centres, or which contain, as substituent groups, allylic, substituted allylic, vinylic or polyunsaturated species capable of forming radical generating species by reaction with oxygen, ozone or other oxidizing species.

Alternatively, a polymeric precursor including the varieties described may contain as substituents carboxy, hydroxy, isocyanate, acyl or aroyl halides, or other groups or their derivatives, which can be condensed with hydroperoxides, substituted peroxides or substituted azo derivatives or other initiators to yield a polymeric adduct capable of acting as a radical generator. Although in most cases the presence of polar groups in the coating material will adequately bond the coating polymer to the pigment or filler surface by strong physical forces, improved chemical bonding can be achieved by incorporation in the coating material of reactive substituents, including hydroxy, amino, isocyanate, carboxylic or sulphonic acids or their derivatives or alkoxysilyl groups, either by copolymerisation of a suitable monomer, such as acrylic acid or methacrylic, cyanoethyl or sulphoethyl acrylate or methacrylate, substituted styrene, triethoxyvinylsilane, tetraethylsilicate, or trimethoxysilylpropyl methacrylate, during formation of the coating polymer, or by subsequent reaction.

EXAMPLE 1.

Aluminium sulphate dissolved in water (50 ml, 10%) was added to a sample of titanium dioxide (50g) dispersed in water (150 ml). The slurry was stirred for 1 hour and then ammonium hydroxide added slowly to bring the pH to 7. The slurry was filtered, and the treated titanium dioxide washed twice with water and then dried overnight at C. This TiO was then treated as follows:

1 l a. slurried with dilute sulphuric acid to give a pH of the slurry of 2.5. The slurry was stirred for mins, filtered and the titanium dioxide washed twice with water and then dried overnight at 120C.

b. The dried material was then calcined at 450C for 2 hours. 1 c. The material was treated with dilute sulphuric acid solution to give a slurry of pH 3.5. The slurry was stirred for 1 hour, filtered and the TiO washed twice with water and dried at 120C overnight.

EXAMPLE 2 A sample of titanium dioxide (50g) was dispersed in water (100 ml) and then a solution of sodium aluminate (3.2g in 50 ml water) was added with stirring. After stirring for 10 minutes a solution of sodium silicate (5.6g in 50 ml water) was added and stirring continued for a further 10 minutes. The pH of the slurry was then adjusted to pH7 with 2 N sulphuric acid and allowed to stand overnight. The slurry was filtered and the coated titanium dioxide washed twice with water and dried at 120C. This coated material was then treated in the following ways:

a. slurried with dilute sulphuric acid to give a slurry of pH 3.5 to 4.0. After stirring for 1 hour, the slurry was filtered and the TiO washed twice with water and then dried. The dried material was then calcined at 450C for 2 hours.

b. heated at 450C for 2 hours then treated as in (a) c. treated as in (a) but with aluminum potassium sulphate (5% solution) instead of 1N H 80 d. treated as in (a) but with acetic acid solution instead of dilute sulphuric acid.

e. treated as in (a) but final calcining was at 300C.

f. treated as in (a) but final calcining was at 650C.

EXAMPLE 3 Coated titanium dioxide samples were prepared in the same manner as in Example 2 but the pH was adjusted to pH7 with 2N sulphuric acid after the addition of the sodium aluminate and before the addition of the sodium silicate.

EXAMPLE 4 a. A commercially coated titania with about 4.1% aluminium as A1 0 and 8.5% silica as SiO was treated as in example 2(a).

b. A commercially coated titania with about 2.0% aluminium as A1 0 and 0.75 silica as SiO was treated as in example 2(a).

The materials obtained from Examples 1 4 were tested according to the method described earlier, to determine the acid sites produced. The following results were obtained.

.,7 -continued Example No. Total acidity Acidity to (millimoles amine/g pKa value pigment X 10". to of 1.5 pKa value of 2.8)

3d 13 10 4a 15 l2 4b 10 8 REACTION WITH BASES EXAMPLE 5 The product obtained in example 2(a), was placed in a high-speed mixer (sorvalOmnimixer). Tallowamine (0.5% by weight on the titanium dioxide) was added and the mixer run on speed 4 for 2 minutes. A further 0.5% by weight of tallowamine was then added and the mixer run at speed 4 for 2 minutes and then speed 2 for 2 minutes.

In separate experiments, the acidic titanium dioxide was treated similarly with octylamine (0.5% by weight on the titanium dioxide) and with hexanol (0.4% by weight). g

The materials produced were tested to determine their improvement in viscosity as suspensions, by measuring the viscosity of a 20% w/w slurry of each of the materials in Ondina oil 33 using a Brookfield LVT Vixcometer.

For comparison, a commercially available silicaalumina coated titania was treated as described in this example, with tallowamine, octylamine and hexanol. The same commercial material was also first heated at l 10C for 4 hours and then treated with the three bases.

The viscosities (in poise) obtained are shown below:

Tallowamine Octylamine hexanol Acidic TiO 7.6 8.8 14.1 Commercial TiO 23 22 20 24.4 25

Commercial TiO /C 16.4

V POLYMER COATING AND ITS OXIDATION EXAMPLE 6 A sample of acidic titania (prepared as in Example 1(a)) was slurried with dry hexane containing 1% of styrene monomer. The slurry was stirred for 30 minutes and the titania was filtered off and dried. The dry titania was then tumbled while a stream of 1% ozone in air was passed at 3 l/min. This treatment was carried out EXAMPLE 7 An acidic titania (prepared as in Example 2(a)) was fluidized in a bed, with dry air. lnto the stream was introduced a 1:1 by weight mixture of divinyl benzene and sytrene monomers, so that 2% by weight (based on the titania) was passed through the system every EXAMPLE 8 A sample of acidic titania (100 g, prepared as in Example 4(a) was stirred in a tumble mixerunder an atmosphere of nitrogen. Nitrogen saturated with isoprene vapour at'30C was introduced into the mixer atmosphere. The mixing was continued for 1 hour, the isoprene being replenished as it was used. Four drops (0.2g) of dimethylaminoethylmethacrylate was then added and the'mixture stirred-for a further one-half hour. The thus treated titania was tumbled in a stream of air for two weeks at C and the peroxide content analysed at'daily'intervals. The peroxide content rose to 0.1 m moles/gm within 3 days and remained at this value for the rest of the time. Carbon analysis of this Commcrical, aluminium silicate -continued Yield Stress m.

Example 4(a) "Acidic TiO (dried) 22| Example 4(a). *Acidic TiO- 0:57: w/w hcptanol added to pigment dispersion (undried) 1' 435 2.40 2.52 0.5% w/w heptanol added to pigment (in omnimix) before dispersion (undried) Examp'le'4(a) Acidic" TiO 0.5% w/w dccylamine added to pigment (,in omnimix) before dispersion (undried) coated TiO 0.5% w/w heptanol added to pigment (in omnimix) before dispersion (undried) 225 2.25

Example 8 Acidic" isoprene coated TiO EXAMPLE 1' 0 -.Similar viscosity measurements of 30% w/w slurries .of thesev pigments wereobtained in Linseed oil and are shown'in the following table.

Y ield Stress Commercial, uncoated TiO (undried) dyne.cm 0.6 poise Commercial, aluminium silicate TiO (Example 4' undried) 320 0.56 Example 4(a) Acidic TiO 1 (undried) 4 4 .056 Example 4 (a) Acidic TiO (dried) l.()' =06 Example 4(a) Acidic" TiO 0.5% w/w heptanol added to pigment I dispersion (undried) f 18.5 0.6] .Example 8 Acidic isoprene coated TiO (undried) l.l7 0.67

mater al indicated a coating value of about 4.7% polyv x isoprene. (At C the peroxide content rose to the EXAMPLE 1 1 same maximum in eight hours).

EXAMPLE 9 for 12 hours were used. The yield stresses (in dyne.

cmand viscosity at infinite shear 1 (in poise) are shown below:

Yield Stress 1 Commercial. uncoated l'iO l500 (undried) Commercial, uncoated TiO 350 L60 (dried) Commercial, aluminium silicate coated TiO: Example 4 (undried) 1420 2.72 Commercial. aluminum silicate coated TiO Example 4 (dried) 806 2.25 Example 4(a), Acidic" TiO (undried) 565 2.40

Example 4(a). cidic" TiO Some of the materials tested in previous Examples were also compared todetermine differences in the uptake of moistureon to the surface. This was measured by the average weight gain of four, lg samples when exposed to a' relative humidity of 60% for 2 hours. The resultsobtained were:

% gain in weight Acidic titanium dioxide (Example 8) isoprene coated 0.27 Commercial TiO aluminium silicate coated 1.72 Acidic titanium dioxide/W; tallow 0.65

amine Acidic titanium dioxide/0.5% octylamine 0.82 Commercial TiO, l00C/l% Tallowamine 0.93 Commercial TiO l l0C/0.5% octylamine 1.3%

15 of this invention are those described in our copending Australian Patent applications Nos. PA 3980/71, PA 3982/71, PA 3984/71 and PA3986/7l, all filed 10th February, 1971' These materials are prepared in a manner similar to the pigments of this invention.

The present invention includes extended pigments of the above type and coatings and plastics materials containing them and all other modifications which fall within its spirit and scope.

What we claim is:

l. A process for the production of polymer-coated titanium dioxide particles comprising polymerizing a monomer capable of acid-catalysed polymerization in the presence of titanium dioxide particles coated with an aluminium compound and having on their surface a number of acidic sites with a pKa value below 2.8.

2. A process as claimed in claim 1 wherein the particles are also coated with a compound of silicon.

3. A process as claimed in claim 1 wherein the monomer is selected from the group consisting of styrene, substituted styrenes, aliphatic and aromatic olefines with more than one double bond, vinyl ethers, substituted vinyl ethers, N-vinyl carbazole, divinyl benzene, vinyl ketone, indenes, and coumarones.

4. A process as claimed in claim 1 where the polymerformed from the monomer is oxidized in such a manner as to form reactive groups on the surface of the polymer selected from those consisting of peroxide, hydroperoxide and ozonide groups.

5. A process as claimed in claim 4 where the polymercoated titanium dioxide particles, before oxidation, are exposed to the vapor of a polar compound selected from the group consisting of water, an alcohol or an ester 6. A process for producing polymer-coated titanium dioxide particles comprising a. forming an aqueous slurry of titanium dioxide particles coated with a compound of aluminium, saidslurry having a pH of less than 5,

b. recovering titanium dioxide particles from the slurry while maintaining the pH value below 5,

c. heatingthe particles for a sufficient period of time i to form upon their surface a number of acidic sites v having a pKa value below 2.8, l

d. polymerizing a monomer capable of acid-catalysed polymerization in-the presence of the said titanium dioxide particles from (c), and t e. recovering polymer-coated titanium dioxide particles;

7. A process as claimed in claim 6 wherein the recovered polymer-coated titanium dioxide particles are oxidized to form upon their surface reactive groups selected from those consisting of peroxide, hydroperoxide and ozonide groups. v

8; Titanium dioxide particles coated with a compound of aluminium and apolymer and having on the surface of the polymer reactive groups selected from those consisting of peroxide, hydroperoxide and 020- nide groups.

9. Titanium dioxide particles as claimed in claim 8 which are also coated with a compound of silicon.

vl0. The titanium dioxide particles of claim 8 wherein said coated polymer is formed bypolymerizing a monomer capable of undergoing acid catalyzed polymerization in the presence of titanium dioxide particles coated with aluminium and having on their surface a number of acidic sites and a pKa value below 2.8.

11. A process for producing polymer-coated titanium dioxide particles comprising a. forming an aqueous slurry of titanium dioxide particles coated with a compound of aluminium, said slurry having a pH of less than 5,

b. recovering titanium dioxide particles from the slurry while maintaining the pH value below 5,

c. heating the particles for a sufficient period of time to form upon their surface a number of acidic sites having a pKa value below 2.8,

d. reacting the acidic sites of said particles with an ethylenically unsaturated organic base capable of undergoing further ethylenic addition polymerization, and i g I e. polymerizing an ethylenically unsaturated monomer in the presence of the particles obtained in (d). 

1. A PROCESS FOR THE PRODUCTION OF POLYMER-COATED TITANIUM DIOXIDE PARTICLES COMPRISING POLYMERIZING A MONOMER CAPABLE OF ACID-CATALYSED POLYMERIZATION IN THE PRESENCE OF TITANIUM DIOXDE PARTICLES COATED WITH AN ALUMINUM COMPOUND AND HAVING ON THEIR SURFACE A NUMBER OF ACIDIC SITES WITH A PKA VALUE BELOW 2.8.
 2. A process as claimed in claim 1 wherein the particles are also coated with a compound of silicon.
 3. A process as claimed in claim 1 wherein the monomer is selected from the group consisting of styrene, substituted styrenes, aliphatic and aromatic olefines with more than one double bond, vinyl ethers, substituted vinyl ethers, N-vinyl carbazole, divinyl benzene, vinyl ketone, indenes, and coumarones.
 4. A process as claimed in claim 1 where the polymer formed from the monomer is oxidized in such a manner as to form reactive groups on the surface of the polymer selected from those consisting of peroxide, hydroperoxide and ozonide groups.
 5. A process as claimed in claim 4 where the polymercoated titanium dioxide particles, before oxidation, are exposed to the vapor of a polar compound selected from the group consisting of water, an alcohol or an ester.
 6. A process for producing polymer-coated titanium dioxide particles comprising a. forming an aqueous slurry of titanium dioxide particles coated with a compound of aluminium, said slurry having a pH of less than 5, b. recovering titanium dioxide particles from the slurry while maintaining the pH value below 5, c. heating the particles for a sufficient period of time to form upon their surface a number of acidic sites having a pKa value below 2.8, d. polymerizing a monomer capable of acid-catalysed polymerization in the presence of the said titanium dioxide particles from (c), and e. recovering polymer-coated titanium dioxide particles.
 7. A process as claimed in claim 6 wherein the recovered polymer-coated titanium dioxide particles are oxidized to form upon their surface reactive groups selected from those consisting of peroxide, hydroperoxide and ozonide groups.
 8. Titanium dioxide particles coated with a compound of aluminium and a polymer and having on the surface of the polymer reactive groups selected from those consisting of peroxide, hydroperoxide and ozonide groups.
 9. Titanium dioxide particles as claimed in claim 8 which are also coated with a compound of silicon.
 10. The titanium dioxide particles of claim 8 wherein said coated polymer is formed by polymerizing a monomer capable of undergoing acid catalyzed polymerization in the presence of titanium dioxide particles coated with aluminium and having on their surface a number of acidic sites and a pKa value below 2.8.
 11. A process for producing polymer-coated titanium dioxide particles comprising a. forming an aqueous slurry of titanium dioxide particles coated with a compound of aluminium, said slurry having a pH of less than 5, b. recovering titanium dioxide particles from the sLurry while maintaining the pH value below 5, c. heating the particles for a sufficient period of time to form upon their surface a number of acidic sites having a pKa value below 2.8, d. reacting the acidic sites of said particles with an ethylenically unsaturated organic base capable of undergoing further ethylenic addition polymerization, and e. polymerizing an ethylenically unsaturated monomer in the presence of the particles obtained in (d). 